BERLIN (Reuters) - Chancellor Angela Merkel's conservatives face a heavy defeat in a regional election on Sunday that would extend their losing streak to six states this year and weaken her hand before a crucial euro zone vote in parliament.

Opinion polls show Merkel's Christian Democrats (CDU) will win about 22 percent of the vote in the city-state of Berlin on Sunday and come a distant second to the center-left Social Democrats (SPD), who are forecast to win about 31 percent and stay in power with either the Greens or the Left party.

The polls in Berlin opened at 8 a.m. and the first exit polls will be announced immediately after polls close at 6 p.m. It was raining heavily in Berlin on Sunday morning and weather forecasts called for more showers.

Merkel, under fire for her hesitant leadership in the euro zone crisis, is halfway through a four-year term. But election setbacks for her CDU have hurt her standing before the vote on euro zone measures in parliament at the end of September.

"I never really liked Merkel anyway, and she doesn't seem to be doing a good job leading at all," said Claudia Barre, 31, an insurance sales woman on her way to vote.

There could be more bad news for Merkel's center-right coalition in Berlin if her junior coalition partners, the Free Democrats (FDP), fail to win at least five percent of the vote and are ejected from the state assembly.

It would be the fifth time in seven elections this year that the FDP had failed to win five percent and could increase pressure on the party, which won a record 14.6 percent in the 2009 federal election, to remove Foreign Minister Guido Westerwelle. Polls show the FDP at about 2 percent.

Another surprise on Sunday could be the performance of the Pirate Party, a German branch of a party that emerged in Sweden five years ago to campaign for reform of copyright and better privacy in the Internet age. Polls show it winning 9 percent.

The SPD, in opposition at the national level since 2009, wants likely re-election in Berlin on Sunday to build up momentum to oust Merkel in the next federal election in 2013.

The SPD has ousted or helped defeat the CDU in Hamburg and Baden-Wuerttemberg this year and remained in power elsewhere.

The CDU has lost five of six regional votes. A bad result in Berlin, Germany's largest city with 3.4 million, would add to Merkel's woes before a Bundestag vote on September 29 to give the European Financial Stability Fund (EFSF) more powers.